Diagonal collection of oblique imagery

ABSTRACT

A vehicle collects oblique imagery along an intercardinal nominal heading using rotated camera-groups with distortion correcting electronic image sensors that align projected pixel columns or rows with a pre-determined direction on the ground, thereby improving collection quality, efficiency, and/or cost. In a first aspect, the camera-groups are rotated diagonal to the nominal heading. In a second aspect, the distortion correcting electronic image sensors align projected pixel columns or rows with a pre-determined direction on the ground. In a third aspect, the distortion correcting electronic image sensors are rotated around the optical axis of the camera. In a fourth aspect, cameras collect images in strips and the strips from different cameras overlap, providing large-baseline, small-time difference stereopsis.

CROSS REFERENCE TO RELATED APPLICATIONS

Priority benefit claims for this application are made in the accompanying Application Data Sheet, Request, or Transmittal (as appropriate, if any). To the extent permitted by the type of the instant application, this application incorporates by reference for all purposes the following applications, all commonly owned with the instant application at the time the invention was made:

-   -   U.S. Provisional Application Ser. No. 61/786,311, filed Mar. 15,         2013, first named inventor lain Richard Tyrone MCCLATCHIE, and         entitled DIAGONAL COLLECTION OF OBLIQUE IMAGERY.

BACKGROUND

1. Field

Advancements in photogrammetry are needed to provide improvements in performance, efficiency, and utility of use.

2. Related Art

Unless expressly identified as being publicly or well known, mention herein of techniques and concepts, including for context, definitions, or comparison purposes, should not be construed as an admission that such techniques and concepts are previously publicly known or otherwise part of the prior art. All references cited herein (if any), including patents, patent applications, and publications, are hereby incorporated by reference in their entireties, whether specifically incorporated or not, for all purposes.

An example of a camera is an image capturing system that captures imagery using a lens that focuses light on at least one Petzval surface (e.g. a focal plane), and captures an image with at least one image sensor on the Petzval surface. A focal plane is an example of a planar Petzval surface. In general, Petzval surfaces are not required to be planar and may be curved due to the design of the lens. Examples of image sensors include film and electronic image sensors. Examples of electronic image sensors include Charge Coupled Device (CCD) sensors and Complementary Metal-Oxide Semiconductor (CMOS) sensors. An example of an emerging optical axis of a camera is the path along which light travels from the ground at the center of the lens field of view to arrive at the entrance to the camera. The light path inside the camera may be folded with reflecting surfaces, but eventually light arriving along the emerging optical axis will converge at the center of the Petzval surface(s).

Some maps assume a camera perspective looking straight down, called an orthographic (or nadir) perspective. In some embodiments and/or scenarios, this is also the perspective of the captured images used to make these maps (e.g. orthographic imagery). However, orthographic imagery eliminates all information about the relative heights of objects, and information about some surfaces (e.g. the vertical face of a building).

Other maps assume a camera perspective looking down at an angle below the horizon but not straight down, called an oblique perspective. An example of a down angle of a camera is the angle of the emerging optical axis of the camera above or below the horizon; down angles for nadir perspectives are thus 90 degrees; down angles for oblique perspectives are usually 20 to 70 degrees. In some embodiments and/or scenarios, the camera used to capture an oblique perspective is referred to as an oblique camera and the resulting images are referred to as oblique imagery. In some scenarios, oblique imagery is beneficial because it presents information that is useful to easily recognize objects and/or locations (e.g. height and vertical surfaces); information that is typically missing from orthographic imagery.

In some embodiments, the same point on the ground is captured with oblique images captured from multiple perspectives (e.g., 4 perspectives looking at a building, one from each cardinal direction: North, South, East, and West). This is sometimes described as ground-centric collection, and yields ground-centric oblique imagery. In various scenarios, ground-centric aerial oblique imagery is useful, e.g. for assessing the value of or damage to property, particularly over large geographic areas. It is usually a priority in a ground-centric collection program to collect an image of every point in some defined target area for each of the cardinal directions. The capture resolution is measured in distance units on the ground (e.g., 4 inch per pixel) and usually does not vary much between different points in the target area.

In some embodiments, multiple oblique images are captured from a single point, with multiple perspectives (e.g., 4 perspectives looking from a building in each cardinal direction), also known as sky-centric collection. In some scenarios, sky-centric imagery is commonly used to form a panoramic view from a single point. It is usually a priority in a sky-centric collection program to collect a continuous panorama from each viewpoint. Capture resolution is usually measured in angular units at the viewpoint (e.g., 20,000 pixels across a 360 degree panorama).

In various embodiments, a camera-group is a system of one or more cameras that approximately capture the same image (e.g. the optical axes are aligned within 5 degrees of a common reference axis). For example, an ordinary pair of human eyes acts as a 2 camera-group, focusing on a single image. Generally, a camera-group can have an arbitrary number of cameras.

In some embodiments, a camera-set is a system of one or more cameras and/or camera-groups that capture different images. One example of a 2 camera-set is a nadir camera and an oblique camera. Another example of a 4 camera-set is 4 oblique cameras, each pointing in a different cardinal direction. Generally, a camera-set can have an arbitrary number of cameras and/or camera-groups.

An example of the nominal heading of a vehicle is the overall direction of travel of the vehicle. In many scenarios, the instantaneous direction of travel deviates from the nominal heading. For example, an airplane may be flying along a flightpath heading due north, so that the nominal heading is north, while experiencing a wind blowing from west to east. To keep the plane on the flight path, the pilot will point the plane into the wind, so that the instantaneous heading is many degrees west of north. As another example, a car is driving down a straight road that runs from south to north and has several lanes. The nominal heading is north. However, to avoid hitting an obstacle, the car may changes lanes, instantaneously moving northwest, rather than strictly north. Despite this instantaneous adjustment, the nominal heading is still north. In contrast, when the car turns 90 degrees from north to travel west, the nominal heading is now west.

An example of a plan angle of an oblique camera on a vehicle is angle between the nominal heading of the vehicle and the emerging optical axis of the camera projected onto the ground plane; plan angles vary from 0-360 degrees. Some cameras are mounted on stabilization platforms so that the camera maintains its plan angle even as the instantaneous heading changes. Some cameras are mounted directly to the vehicle. Note that a vehicle may have a nominal heading, even when stopped, e.g. a helicopter with a flightpath due north could stop periodically, but would still have a nominal heading of due north.

Camera-sets used for sky-centric collection expend far more film (and later pixels) on ground points that the vehicle travels directly over, compared to ground points off to the side of the vehicle's path. When aerial photography and photogrammetry began to use airplanes, it became important to use less film to reduce costs. Some camera-sets removed the forward- and rear-facing oblique cameras of the earlier designs, and used a nadir camera and two oblique cameras pointing to the side (e.g. all emerging optical axes approximately perpendicular to the nominal heading of the airplane). While flying in a straight line and capturing overlapping images, these camera-sets capture the same amount of ground area with the same resolution as the more complex panoramic cameras and/or camera-sets, but with less film.

The extent of coverage in the direction of flight (sometimes described as in track) is primarily determined by the distance of flight. The extent of coverage orthogonal to the direction of flight (sometimes described as cross track) is primarily determined by the plane's altitude and the design of the camera. The extent of coverage in the cross track direction is sometimes called the swath. One benefit of a camera-set with both an oblique camera and a nadir camera is achieving greater swath without complex lens designs (e.g., a single large FOV fisheye).

In some sky-centric collection scenarios, the vehicle is maneuvered until the objects of interest are in view. For some ground-centric collection scenarios, the vehicle moves through a pattern which gives an opportunity to capture each point of interest on the ground from every required direction. In various embodiments, a Maltese Cross camera-set is moved in a path consisting of parallel lines (e.g. flight lines of an airplane) that run in a north-south or east-west direction. As the vehicle moves along the flight lines, the images captured by any particular camera can be superposed to form a long continuous strip of coverage. The length of this strip will be approximately the length of the flight line, and the width of this strip is known as the swath.

FIG. 1 conceptually illustrates an isometric view of selected prior art details of an airplane 102 with a Maltese Cross style oblique camera-set. The sensor fields of view of the forward 104, right 106, back 108, and left 110 oblique cameras are shown, projected onto the ground. The emerging optical axes of the cameras (respectively 112, 114, 116, and 118) have 45 degree down angles. Down Angle 122 is the angle formed between the Emerging Optical Axis 114 and its projection 120 to a plane parallel to the ground. For clarity, the other down angles are omitted from the illustration.

FIG. 2 conceptually illustrates a plan view of selected prior art details of the field of view of a single example camera of a Maltese Cross camera-set. The conical field of view projects from the camera aperture 208 to an ellipse 202 on the planar surface, with the longer major axis of the ellipse pointing away from the center of the camera. The image formed by the lens is a circle 210, which is shown at the left at a larger scale, and looking down the lens optical axis. The image sensor is an inscribed rectangle 212 that projects to a trapezoid 204 on the surface, because of the down angle of the camera. The image sensor is a rectangular array of pixels arranged in rows 220 and columns 216. The light rays 206 corresponding to the four corners of the image sensor are also shown. These light rays come from the ground up through the lens to the sensor. The pixels of the image sensor are projected onto the ground, forming projected rows 218 and projected columns 214. In this example, the rectangular image sensor is 24 mm by 36 mm, the focal length is 100 mm, and the camera altitude above the surface is 1000 meters. The resulting trapezoid is 455 meters wide at its base and 579 meters wide at its top.

FIG. 3 conceptually illustrates a plan view of selected prior art details of capturing oblique imagery via a Maltese Cross camera-set. In various embodiments, the nominal heading of the vehicle 301 is a cardinal direction (e.g. North, South, East, West). The camera-set includes four oblique cameras, with 0, 90, 180, and 270 degree plan angles. For conceptual clarity, the emerging optical axes are drawn in FIG. 3 with a 3 degree offset. Each camera has the same focal length and sensor size as the example camera in FIG. 2. However, the left and right cameras have the longer 36 mm dimension of the sensors aligned with the nominal heading. The projected FOV ellipses of the cameras 304, 308, 312, and 316 contain the projected sensor FOV trapezoids, respectively 302, 306, 310, and 314. Several captured images 320 of the projected FOV trapezoids are shown. The captured images from a single camera in a single flight line form a continuous strip, and there is significant forward overlap between images in the strip (e.g., at least 50% and typically 60% overlap between sequentially captured images).

The collection swath of a camera must fit within the projected FOV ellipses. In FIG. 3, the forward and back swaths are constrained by the minor axis of the front and back FOV ellipses; the side-facing swaths are constrained by the major axis of the side-facing FOV ellipses, which are significantly larger. In this example, the sensor FOVs of the left and right cameras are 487 meters wide, and the sensor FOVs of the front and back cameras are 458 meters wide (distance 355).

The swaths of the front- and rear-facing cameras are also significantly smaller than the separation between the swaths of the side-facing cameras. The front-facing camera swath is between edges 352 and 354, and as noted is 458 meters wide. The inner edges of the side facing swaths are denoted by edges 362 and 364, and the space between them 365 is 1571 meters.

FIG. 4 conceptually illustrates selected prior art details of an example flight plan for capturing oblique imagery covering Alexandria County, Virginia, using the Maltese Cross camera-set of FIG. 3. The flight plan 401 is arranged in 25 flight lines (e.g., 402) with nominal headings east or west, separated by 24 turns (e.g., 403) and captures oblique images that are oriented north, south, east and west. The total flight distance is 264 kilometers.

To capture the views offered by the front and rear facing cameras for every point of interest on the ground, the vehicle's flight lines must be closer together than the swath of the front and rear facing cameras. In the flight plan depicted in FIG. 4, the flight line pitch is 340 meters, so that there is 25% horizontal overlap between adjacent strips of imagery.

SYNOPSIS

The invention may be implemented in numerous ways, including as a process, an article of manufacture, an apparatus, a system, a composition of matter, and a computer readable medium such as a computer readable storage medium (e.g. media in an optical and/or magnetic mass storage device such as a disk, or an integrated circuit having non-volatile storage such as flash storage) or a computer network wherein program instructions are sent over optical or electronic communication links. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. The Detailed Description provides an exposition of one or more embodiments of the invention that enable improvements in performance, efficiency, and utility of use in the field identified above. The Detailed Description includes an Introduction to facilitate the more rapid understanding of the remainder of the Detailed Description. The Introduction includes Example Embodiments of one or more of systems, methods, articles of manufacture, and computer readable media in accordance with the concepts described herein. As is discussed in more detail in the Conclusions, the invention encompasses all possible modifications and variations within the scope of the issued claims.

In some embodiments, the camera designer chooses whether to align either the projected rows or projected columns of the image sensor with the direction of flight. More frequently the column vector, projected onto the ground, is aligned near the nominal heading, which leaves the row vector, projected onto the ground, aligned as near as practical to the cross-track direction. An example of a twist angle of an image sensor is the angle between the image sensor row vector and a vector at the Petzval surface, orthogonal to the optical axis, and parallel to the ground plane (sometimes referred to as the horizontal vector).

In one embodiment, the vehicle carries at least four oblique cameras, at least one pointed approximately in each of the four diagonal directions from the nominal heading of the vehicle (e.g. 45, 135, 225 and 315 degree plan angles). The flight lines of the collection flight plan are in the intercardinal directions (northeast, northwest, southeast, or southwest).

In another embodiment, the vehicle carries at least four oblique cameras with distortion correcting electronic image sensors. The electronic image sensors behind each lens have a twist angle such that the columns or rows of the projected sensor field of view are approximately aligned with the nominal heading. In some embodiments, the four oblique cameras are positioned in a Maltese Cross configuration (e.g., plan angles of approximately 0, 90, 180, and 270 degrees), while in other embodiments the four oblique cameras are positioned diagonally (e.g. 45, 135, 225 and 315 degree plan angles).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 conceptually illustrates an isometric view of selected prior art details of an airplane with a Maltese Cross style oblique camera-set.

FIG. 2 conceptually illustrates a plan view of selected prior art details of the field of view of a single example camera of a Maltese Cross camera-set.

FIG. 3 conceptually illustrates a plan view of selected prior art details of capturing oblique imagery via a Maltese Cross camera-set.

FIG. 4 conceptually illustrates selected prior art details of an example flight plan for capturing oblique imagery covering Alexandria County, Virginia, using the Maltese Cross camera-set of FIG. 3.

FIG. 5 conceptually illustrates a plan view of selected details of an embodiment of capturing oblique imagery via a camera-set with emerging optical axes rotated in plan.

FIG. 6 conceptually illustrates a plan view of selected details of an embodiment of capturing oblique imagery via a camera-set with rotated emerging optical axes and distortion correcting sensors.

FIG. 7 conceptually illustrates selected details of an example flight plan for embodiments of capturing oblique imagery covering Alexandria County, Virginia, using the camera-set of FIG. 6.

FIG. 8A conceptually illustrates selected details of the FOV of the forward camera from two adjacent flight lines for a Maltese Cross camera-set capturing oblique imagery.

FIG. 8B conceptually illustrates selected details of the FOV of the forward-right camera from two adjacent flight lines for an embodiment of capturing oblique imagery via a camera-set with rotated emerging optical axes and distortion correcting sensors.

FIG. 9 conceptually illustrates a plan view of an embodiment of capturing oblique and nadir imagery via a camera-set with rotated emerging optical axes and distortion correcting sensors, where the nadir and oblique swaths overlap slightly.

FIG. 10 conceptually illustrates a plan view of selected details of embodiments of a vehicle traveling diagonally.

FIG. 11 conceptually illustrates a plan view of selected details of embodiments of a vehicle with a rotated oblique camera-set.

FIG. 12A conceptually illustrates selected details of embodiments of an oblique camera with an electronic image sensor that projects to a distorted sensor field of view.

FIG. 12B conceptually illustrates selected details of embodiments of an oblique camera with a non-uniform distortion correcting electronic image sensor that projects to a corrected sensor field of view.

FIG. 13 conceptually illustrates selected details of embodiments of a diagonal oblique camera with a rotated distortion correcting electronic image sensor that projects to a partially corrected sensor field of view.

FIG. 14 conceptually illustrates selected details of embodiments of a diagonal oblique camera with a rotated array of rotated distortion correcting electronic image sensors that projects to an array of partially corrected sensor fields of view.

DETAILED DESCRIPTION

A detailed description of one or more embodiments of the invention is provided below along with accompanying figures illustrating selected details of the invention. The invention is described in connection with the embodiments. The embodiments herein are understood to be merely exemplary, the invention is expressly not limited to or by any or all of the embodiments herein, and the invention encompasses numerous alternatives, modifications, and equivalents. To avoid monotony in the exposition, a variety of word labels (including but not limited to: first, last, certain, various, further, other, particular, select, some, and notable) may be applied to separate sets of embodiments; as used herein such labels are expressly not meant to convey quality, or any form of preference or prejudice, but merely to conveniently distinguish among the separate sets. The order of some operations of disclosed processes is alterable within the scope of the invention. Wherever multiple embodiments serve to describe variations in process, method, and/or program instruction features, other embodiments are contemplated that in accordance with a predetermined or a dynamically determined criterion perform static and/or dynamic selection of one of a plurality of modes of operation corresponding respectively to a plurality of the multiple embodiments. Numerous specific details are set forth in the following description to provide a thorough understanding of the invention. The details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of the details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured.

Introduction

This introduction is included only to facilitate the more rapid understanding of the Detailed Description; the invention is not limited to the concepts presented in the introduction (including explicit examples, if any), as the paragraphs of any introduction are necessarily an abridged view of the entire subject and are not meant to be an exhaustive or restrictive description. For example, the introduction that follows provides overview information limited by space and organization to only certain embodiments. There are many other embodiments, including those to which claims will ultimately be drawn, discussed throughout the balance of the specification.

Example Embodiments

In concluding the introduction to the detailed description, what follows is a collection of example embodiments, including at least some explicitly enumerated as “ECs” (Example Combinations), providing additional description of a variety of embodiment types in accordance with the concepts described herein; these examples are not meant to be mutually exclusive, exhaustive, or restrictive; and the invention is not limited to these example embodiments but rather encompasses all possible modifications and variations within the scope of the issued claims.

EC1) A method comprising:

-   -   operating a vehicle comprising one or more respective         camera-groups enabled to capture oblique imagery via electronic         image sensor technology;     -   configuring each of the respective camera-groups with a         respective pre-determined plan angle range; and     -   establishing the nominal heading as corresponding to a cardinal         direction plus a pre-determined first angular offset, where the         first angular offset is between 10 and 80 degrees, and capturing         oblique imagery in some cardinal direction with at least one         camera of the one or more of the respective camera-groups.

EC2) The method of EC1 wherein at least one of the one or more respective camera-groups comprises a single camera.

EC3) The method of EC1 wherein at least one of the one or more respective camera-groups comprises multiple cameras.

EC4) The method of EC1 wherein the respective camera-groups comprise N particular camera-groups, each of the N particular camera-groups is associated with a unique integer K between 0 and (N−1) inclusive, and the respective pre-determined plan angle range of the particular camera-group is (180+360*K)/N degrees plus a first pre-determined angular offset range.

EC5) The method of EC4 wherein the first pre-determined angular offset range is between minus 120/N and plus 120/N degrees.

EC6) The method of EC5 wherein N is four or eight.

EC7) The method of EC1 further configuring a particular electronic image sensor of a particular one of the respective camera-groups in an orientation to reduce angular separation between the nominal heading and one of a projected pixel column and a projected pixel row of the particular electronic image sensor below a pre-determined separation threshold.

EC8) The method of EC7 wherein the pre-determined separation threshold is 30 degrees.

EC9) The method of EC7 wherein the configuring comprises rotating the particular electronic image sensor around the optical axis of a camera of the camera-group.

EC10) The method of EC1 wherein the vehicle further comprises at least one nadir camera-group enabled to capture nadir imagery.

EC11) The method of EC10 wherein a sensor field of view within the nadir camera-group overlaps a sensor field of view within at least one of the respective camera-groups.

EC12) The method of EC1 wherein the vehicle is one or more of an aircraft, an airplane, a lighter-than-air craft, a space-craft, a helicopter, a satellite, a car, a truck, a land-based vehicle, a ship, a boat, a barge, a canoe, a submersible, and a submarine.

EC13) The method of EC12 wherein the vehicle is unmanned or manned.

EC14) The method of EC1 wherein at least one electronic image sensor of the respective camera-groups is enabled to capture infrared radiation.

EC15) The method of EC1 wherein at least one of the respective cameras comprises an electronic image sensor.

EC16) The method of EC1 wherein at least one camera of the respective camera-groups comprises at least one partially reflective element and a plurality of Petzval surfaces.

EC17) The method of EC1 wherein at least one camera of the respective camera-groups comprises a staggered array of electronic image sensors at a Petzval surface or a butted array of electronic image sensors at a Petzval surface.

EC18) The method of EC1 wherein at least one of the respective camera-groups comprises a plurality of cameras with parallel or nearly parallel lenses, each camera comprising an array of electronic image sensors at its Petzval surface(s), such that the projected fields of view of the sensors overlap.

EC19) The method of EC1 wherein a Petzval surface for at least one camera of the respective camera-groups comprises at least two area-format electronic image sensors or at least two line-format electronic image sensors.

EC20) The method of EC1 wherein the cardinal direction is a true cardinal direction or a magnetic cardinal direction.

EC21) The method of EC1 wherein the pre-determined first angular offset is within a second pre-determined angular offset range.

EC22) The method of EC21 wherein the respective camera-groups comprise N particular camera-groups and the second pre-determined angular offset range is between 300/(2*N) and 420/(2*N) degrees.

EC23) The method of EC1 wherein the capturing comprises capturing a plurality of images from at least a first one of the respective camera-groups.

EC24) The method of EC23 wherein the plurality of images are captured sequentially in a strip.

EC25) The method of EC24 wherein the plurality of images comprises first, second, and third contiguously obtained images, the second image overlaps by at least 50% with the first image, and overlaps by at least 50% with the third image.

EC26) The method of EC24 wherein the strip of images is a first strip of images, the capturing further comprises capturing a plurality of images from at least a second one of the respective cameras as a second strip of images, and the first strip and the second strip overlap with each other.

EC27) The method of EC26 wherein the first image strip is captured at a first period in time and the second image strip is captured at a second period in time and the first period in time is distinct from the second period in time.

EC28) The method of EC26 wherein a first image in the first image strip overlaps with a second image in the second strip and the first image is captured at a first period in time and the second image is captured at a second period in time and the first period in time is distinct from the second period in time.

EC29) The method of EC10 wherein the capturing comprises capturing a first plurality of images from at least a first one of the respective camera-groups and a second plurality of images from at least one camera of the nadir camera-group.

EC30) The method of EC29 wherein the first plurality of images is captured sequentially in a first image strip and the second plurality of images is captured sequentially in a second image strip.

EC31) The method of EC30 wherein the first and second image strip overlap.

EC32) The method of EC31 wherein a first image in the first image strip overlaps with a second image in the second strip and the first image is captured at a first period in time and the second image is captured at a second period in time and the first period in time is distinct from the second period in time.

EC33) The method of EC1 wherein the capturing is performed by all of the respective camera-groups.

EC34) A method comprising operating a vehicle comprising one or more respective camera-groups enabled to capture oblique imagery via a distortion correcting electronic image sensor.

EC35) The method of EC34 wherein the distortion correcting electronic image sensor reduces angular separation between one of projected pixel rows and projected pixel columns of the image sensor and a pre-determined direction on the ground below a pre-determined separation threshold.

EC36) The method of EC35 wherein the pre-determined direction on the ground is a cardinal or intercardinal direction.

EC37) The method of EC35 wherein the pre-determined direction on the ground is a nominal heading.

EC38) The method of EC35 wherein the pre-determined separation threshold is 30 degrees.

EC39) The method of EC35 wherein the one or more respective camera-groups are enabled to capture oblique imagery through a medium other than air.

EC40) The method of EC39 wherein the distortion correcting electronic image sensor reduces distortions introduced at least in part by the medium, changes in the medium, or interfaces to the medium.

EC41) The method of EC39 wherein the medium is one or more of water, oil, and vaccuum.

EC42) The method of EC34 wherein the distortion correcting electronic image sensor comprises an electronic image sensor with a non-zero twist angle.

EC43) The method of EC34 wherein the distortion correcting electronic image sensor comprises a group of electronic image sensors and each electronic image sensor has an individual non-zero twist angle.

EC44) The method of EC34 wherein the distortion correcting electronic image sensor comprises an electronic image sensor with a non-uniform array of pixels.

EC45) The method of EC34 wherein the operating further comprises configuring each of the respective camera-groups with a respective pre-determined plan angle range.

EC46) The method of EC34 wherein the operating further comprises establishing a nominal heading corresponding to a cardinal direction plus a pre-determined first angular offset, where the first angular offset is between 10 and 80 degrees, and capturing oblique imagery with at least one camera of the respective camera-groups.

EC47) The method of EC34 wherein at least one of the one or more respective camera-groups consists of a single camera.

EC48) The method of EC34 wherein at least one of the one or more respective camera-groups comprises multiple cameras.

EC49) The method of EC34 wherein the respective camera-groups comprise N particular camera-groups, each of the N particular camera-groups is associated with a unique integer K between 0 and (N−1) inclusive, and the respective pre-determined plan angle range of the particular camera-group is (180+360*K)/N degrees plus a first pre-determined angular offset range.

EC50) The method of EC49 wherein the first pre-determined angular offset range is between minus 120/N and plus 120/N degrees.

EC51) The method of EC50 wherein N is four or eight.

EC52) The method of EC34 wherein the vehicle further comprises at least one nadir camera-group enabled to capture nadir imagery.

EC53) The method of EC52 wherein a sensor field of view within the nadir camera-group overlaps a sensor field of view within at least one of the respective camera-groups.

EC54) The method of EC34 wherein the vehicle is one or more of an aircraft, an airplane, a lighter-than-air craft, a space-craft, a helicopter, a satellite, a car, a truck, a land-based vehicle, a ship, a boat, a barge, a canoe, a submersible, and a submarine.

EC55) The method of EC54 wherein the vehicle is unmanned or manned.

EC56) The method of EC34 wherein at least one electronic image sensor of the respective camera-groups is enabled to capture infrared radiation.

EC57) The method of EC34 wherein at least one of the respective cameras comprises an electronic image sensor.

EC58) The method of EC34 wherein at least one camera of the respective camera-groups comprises at least one partially reflective element and a plurality of Petzval surfaces.

EC59) The method of EC34 wherein at least one camera of the respective camera-groups comprises a staggered array of electronic image sensors at a Petzval surface or a butted array of electronic image sensors at a Petzval surface.

EC60) The method of EC34 wherein at least one of the respective camera-groups comprises a plurality of cameras with parallel or nearly parallel lenses, each camera comprising an array of electronic image sensors at its Petzval surface(s), such that the projected fields of view of the sensors overlap.

EC61) The method of EC34 wherein a Petzval surface for at least one camera of the respective camera-groups comprises at least two area-format electronic image sensors or at least two line-format electronic image sensors.

EC62) The method of EC46 wherein the cardinal direction is a true cardinal direction or a magnetic cardinal direction.

EC63) The method of EC46 wherein the pre-determined first angular offset is within a second pre-determined angular offset range.

EC64) The method of EC63 wherein the respective camera-groups comprise N particular camera-groups and the second pre-determined angular offset range is between 300/(2*N) and 420/(2*N) degrees.

EC65) The method of EC46 wherein the capturing comprises capturing a plurality of images from at least a first one of the respective camera-groups.

EC66) The method of EC65 wherein the plurality of images are captured sequentially in a strip.

EC67) The method of EC66 wherein the plurality of images comprises first, second, and third contiguously obtained images, the second image overlaps by at least 50% with the first image, and overlaps by at least 50% with the third image.

EC68) The method of EC66 wherein the strip of images is a first strip of images, the capturing further comprises capturing a plurality of images from at least a second one of the respective cameras as a second strip of images, and the first strip and the second strip overlap with each other.

EC69) The method of EC68 wherein the first image strip is captured at a first period in time and the second image strip is captured at a second period in time and the first period in time is distinct from the second period in time.

EC70) The method of EC68 wherein a first image in the first image strip overlaps with a second image in the second strip and the first image is captured at a first period in time and the second image is captured at a second period in time and the first period in time is distinct from the second period in time.

EC71) The method of EC52 wherein the vehicle further comprises at least one nadir camera-group enabled to capture nadir imagery and the capturing comprises capturing a first plurality of images from at least a first one of the respective camera-groups and a second plurality of images from at least one camera of the nadir camera-group.

EC72) The method of EC71 wherein the first pluralities of images is captured sequentially in a first image strip and the second pluralities of images is captured sequentially in a second image strip.

EC73) The method of EC72 wherein the first and second image strip overlap.

EC74) The method of EC73 wherein a first image in the first image strip overlaps with a second image in the second strip and the first image is captured at a first period in time and the second image is captured at a second period in time and the first period in time is distinct from the second period in time.

EC75) The method of EC46 wherein the capturing is performed by all of the respective camera-groups.

System and Operation

FIG. 5 conceptually illustrates a plan view of selected details of an embodiment of capturing oblique imagery via a camera-set with emerging optical axes rotated in plan. For clarity of exposition, the cameras are conceptually identical to the one shown in FIGS. 2 and 3 (e.g. same altitude, same down angle, focal length and image sensor size). In various embodiments, the nominal heading of the vehicle 501 is an intercardinal direction (e.g. NW, NE, SW, SE). In some embodiments, the camera-set includes four oblique cameras, with diagonal emerging optical axes 530, 532, 534, 536. In various embodiments, the camera-set may include an arbitrary number of cameras or camera-groups, e.g., two, three, four, or eight. The emerging optical axes of the cameras are rotated with respect to the nominal heading. In some embodiments, there are four cameras with plan angles of approximately 45, 135, 225 and 315 degrees.

The projected field of view of each camera lens 504, 508, 512, 516 is an ellipse that contains the respective projected sensor FOV 502, 506, 510, 514, which is a trapezoid inscribed in the ellipse. The shape of the camera lens' projected FOV and sensor FOV are due to the down and plan angles of the cameras. An example sensor FOV has a long base 541, a right leg 542, a short base 543 and a left leg 544 and an exposure of the camera will capture the interior of the sensor FOV. Additional captured images of the projected FOV trapezoids are shown, e.g. 520.

In some embodiments, adjacent strips of the ground are captured during adjacent flight lines. To stitch these strips together, portions of the strips are discarded (e.g. jagged edges) to ensure a smooth fit. The non-discarded portions are sometimes called the useful strip. The useful strip of ground captured by the camera corresponding to emerging optical axis 532 is between the boundaries 552 and 554. The swath of the strip (e.g. width of the strip) is less than the shorter base of the trapezoid, due to the spacing between each captured image. To do stereopsis on the captured images, each ground point is captured by two consecutive images. The swath of ground which will be captured by two successive images is between boundaries 556 and 558. A wide swath with stereopsis overlap in a rotated configuration requires the cameras to have a high frame rate (e.g., frame spacing less than one fifth of the swath). As the frame rate gets higher and the stereopsis swath wider, the stereopsis baseline (length of camera translation between successive images) gets smaller, and thus the accuracy of depth perception by stereopsis gets worse.

FIG. 6 conceptually illustrates a plan view of selected details of an embodiment of capturing oblique imagery via a camera-set with diagonal emerging optical axes (e.g., plan angles of approximately 45, 135, 225 and 315 degrees) and distortion correcting sensors. The cameras are conceptually identical to the one shown in FIGS. 2, 3, and 5 (e.g. same altitude, same down angle, focal length and image sensor size, and same plan angles as in FIG. 5). However, the image sensors in the cameras of FIG. 6 correct for the distortion caused by projection onto the ground. The distortion correcting sensor in FIG. 6 is a twisted sensor. The image sensor is rotated around the optical axes of the respective cameras, so that the projected central pixel columns (or pixel rows) of the sensor are roughly aligned to a desired direction on the ground (e.g., nominal heading of the vehicle or a cardinal direction).

A second example of a distortion correcting sensor is a sensor with a non-uniform pixel array. The pixel array is distorted such that the projected pixel columns (or pixel rows) of the sensor are roughly aligned to a desired direction on the ground (e.g., nominal heading of the vehicle or a cardinal direction).

The projected field of view of each camera lens 604, 608, 612, 616 is an ellipse that contains the respective projected sensor FOVs 602, 606, 610, 614, each a trapezium inscribed in the ellipse. The shape of the camera's projected FOV and sensor FOV are due to the down and plan angles of the cameras and the rotation of the sensor around the optical axis of the camera. An example sensor FOV has a long base 641, a right leg 642, a short base 643 and a left leg 644 and an exposure of the camera will capture the interior of the sensor FOV. Additional captured images of the projected FOV trapeziums are shown, e.g. 620.

In various embodiments, the swath for all four cameras are equal. For example, the swath of the camera with emerging optical axis 632 is bounded by inner edge 652 and outer edge 654. The width of this swath is determined by the short base of the trapezoid. In the example of FIG. 6, the swath 655 is 593 meters wide, which is 31% wider than the minimum swath of the example from FIG. 3. The FOVs for cameras on different sides of the vehicle are also spaced closer together. For example, the distance 665 between the inner edge 662 of the front-left swath and inner edge 652 of the front-right swath is 898 meters, which is 43% closer together than the example from FIG. 3. In some embodiments, the larger swath enables the flight lines of the vehicle to be more broadly spaced, reducing the total number of flight lines and total distance traveled by the vehicle, which directly reduces the cost of collecting the oblique imagery. A secondary advantage of more broadly spaced flight lines is that the vehicle speed during turns can be faster, so that less time is spent decelerating and accelerating before and after turns.

Some embodiments have a different number and orientation of the cameras in the camera-set than the conceptual illustration in FIG. 6. Various embodiments have fewer or more cameras (e.g. two, three, four, or eight cameras). Some embodiments have camera orientations that are asymmetric with respect to the nominal heading (e.g. 5 cameras with plan angles of 30, 60, 90, 120, and 150 degrees). In some embodiments, the camera-set includes both cameras with distortion correcting sensors and cameras without distortion correcting sensors (e.g., 8 cameras, four with twisted sensors and plan angles of 45, 135, 225, and 315 degrees, and four with twist angles of zero and plan angles of zero, 90, 180, and 270 degrees).

In some embodiments, a second advantage of rotated cameras with distortion correcting sensors is reducing the distance between the vehicle flight line projected to the ground and the inside edge of the oblique swath. As a result, in some embodiments the amount of extra area that is traveled around the edges of a collection area is reduced. When used for collecting small areas (e.g., less than fifty square kilometers for the example altitude, down angle, plan angle, and sensor size from FIG. 6), this advantage can decrease the cost of collection by a small amount. Additionally, for camera-sets where the nadir camera swath is intended to overlap the oblique swaths, more closely spaced oblique swaths reduce the required swath of the nadir camera, thereby making the nadir camera less expensive.

In various embodiments, a third advantage of rotated cameras with distortion correcting sensors is reducing (e.g., reduced by approximately 35%) the projected ground velocity on the Petzval surface, compared to the side-facing cameras of a Maltese Cross configuration. With a fixed exposure time, a lower projected ground velocity reduces the amount of motion blur and so improves visual quality.

In some embodiments, a fourth advantage of rotated cameras with distortion correcting sensors is improved stereopsis. The swaths captured by the right and left forward rotated cameras are captured a few seconds later by the respective rear rotated cameras, providing large-baseline, small-time-difference stereopsis for both sides of the vehicle. In contrast, a Maltese Cross camera-set can only capture a single large-baseline, short-time-difference stereopsis between the forward, rear, and nadir cameras. Greater collection stereopsis enhances the precision of the 3D ground points triangulated from the collected imagery.

In various embodiments, the rotation of the Petzval surface and image sensors cause the average projected pixel size to slightly increase in size, because the more remote portion of the FOV is better utilized. Equivalently, the average down angle of the pixels is slightly smaller.

In some embodiments, a rotated camera with distortion correcting sensors has a wider swath than the equivalent camera in the forward or rear position of a Maltese Cross (e.g., approximately 30% wider), but the same number of cross-track (e.g., perpendicular to the nominal heading) pixels. So the average cross-track Ground Sample Distance (GSD) is larger (e.g., larger by approximately 30%). But the average in-track (e.g., parallel to the nominal heading) GSD is smaller (e.g., smaller by 30%), so that the average projected pixel area is only slightly larger (e.g., larger by 5% or less). When the camera pixels are resampled into a North-East-West-South grid with uniform GSD north-south and east-west, the resolution differences between Maltese Cross and the rotated cameras is insignificant (e.g., less than 3% linear resolution). The rotated camera's smaller average in-track GSD leads to higher pixel velocity at the Petzval surface (e.g., by about 30%).

FIG. 7 conceptually illustrates selected details of an example flight plan for an embodiment of capturing oblique imagery covering Alexandria County, Virginia, using the camera-set of FIG. 6. The flight plan 701 is arranged in 25 flight lines (e.g., 702) with nominal headings northeast or southwest, separated by 24 turns (e.g., 703) and captures oblique images that are oriented north, south, east and west. FIG. 7 highlights the benefits of embodiments using a rotated camera-set with twisted sensors. The total flight distance is 193 kilometers, compared to 264 kilometers for a Maltese Cross system and will reduce the cost of collection by roughly 27%.

FIG. 8A conceptually illustrates selected details of the FOV of the forward camera from two adjacent flight lines for a Maltese Cross camera-set capturing oblique imagery. Ordinarily there would be some overlap between the image strips swept out by these two swaths, but FIG. 8A omits this overlap for clarity of presentation. The angle 802 is defined by the two camera positions on the two adjacent flight lines, and the point at which the two swaths join. When oblique imagery from the two flight lines are stitched together, visual artifacts such as building lean will be less noticeable if angle 802 is smaller. Thus, minimizing and/or reducing angle 802 enables improved visual quality.

FIG. 8B conceptually illustrates selected details of the FOV of the forward camera from two adjacent flight lines for an embodiment of capturing oblique imagery via a camera-set with rotated emerging optical axes and distortion correcting sensors. Ordinarily there would be some overlap between the image strips swept out by these two swaths, but FIG. 8B omits this overlap for clarity of presentation. The angle 804 between two adjacent flight lines and the joint where the two swaths meet is much smaller due to the geometry of the camera-set and twisted sensors. When oblique imagery from the two flight lines are stitched together, visual artifacts such as building lean will be reduced, because angle 804 is relatively smaller, resulting in superior visual quality.

FIG. 9 conceptually illustrates a plan view of selected details of an embodiment of capturing oblique and nadir imagery via a camera-set with rotated emerging optical axes and distortion correcting sensors, where the nadir and oblique swaths overlap slightly. The oblique cameras are conceptually identical to the one shown in FIG. 6 (e.g. same down and plan angles, focal length and image sensor size).

The projected field of view of the nadir camera lens 974 is a circle that contains the projected sensor FOV 972, which is a square inscribed in the circle. The swath of the nadir camera is bounded by the Projected Sensor FOV. The swath of the camera with emerging optical axis 932 is bounded by inner edge 952 and outer edge 954. Note that the swath of the nadir camera slightly overlaps the swath of the oblique camera, since the Projected Sensor FOV extends past the Inner Edge. However, simultaneous exposures on the nadir camera will not overlap with the oblique camera. This overlap enables high quality imagery and creates a triple baseline stereopsis for any given point in this range (e.g., two oblique shots and a nadir shot).

FIG. 10 conceptually illustrates a plan view of selected details of embodiments of a vehicle traveling diagonally. Nominal Heading Limits 1002 and 1003 form an angular offset range from a Cardinal Direction 1011 (e.g., North). The Vehicle 1000 establishes a Nominal Heading 1001 that falls between the Nominal Heading Limits (e.g., falling within the angular offset range). In some embodiments, the Nominal Heading is enabled to change as long as it stays within the Nominal Heading Limits (e.g., if the camera is mounted to the vehicle without a stabilizer).

In some embodiments of a Vehicle with a camera-set enabled to capture oblique imagery, the Nominal Heading Limits may be determined by the number of camera-groups in the camera-set. In some embodiments with N oblique camera-groups, the Nominal Heading Limits are 300/(2*N) and 420/(2*N) degrees. For example, in an embodiment with 4 oblique camera-groups, the angular offset range is 37.5-52.5 degrees (which can also be expressed as 45±7.5 degrees) from a cardinal direction, meaning that the vehicle can travel diagonally, or approximately Northwest, Northeast, Southwest, or Southeast. In various scenarios, traveling diagonally enhances the productivity of aerial image collection.

FIG. 11 conceptually illustrates a plan view of selected details of embodiments of a vehicle with a rotated oblique camera-set. The Vehicle 1100 has a camera-set with any number of camera-groups enabled to capture oblique imagery (e.g., two, four, seven, eight, etc.), but for clarity only a single camera-group is shown in FIG. 11. Plan Angle 1114 is the angle between Emerging Optical Axis 1111 and Nominal Heading 1101. The Emerging Optical Axis Limits 1112 and 1113 form a plan angle range. The camera-group is configured such that the Emerging Optical Axis falls between the Emerging Optical Axis Limits (e.g., falling within the angular separation range). This enables the Emerging Optical Axes to be biased, as described in a subsequent section. Each camera-group has a different angular separation range and therefore a different configuration. In various scenarios, the Emerging Optical Axis of a camera-group is allowed to vary during oblique image collection (e.g., to accommodate a stabilizer), as long as the Emerging Optical Axis stays within the Emerging Optical Axis Limits.

In some embodiments with a rotated camera-set, the Emerging Optical Axis Limits of each camera-group may be determined by the number of camera-groups in the camera-set. In some embodiments with N camera-groups, the angular separation range of the Kth camera-group is (180+360*K)/N±120/N degrees from the Nominal Heading. For example, in an embodiment with 4 oblique camera-groups the angular separation ranges are 45±30, 135±30, 225±30, and 315±30 degrees from the Nominal Heading. If the established Nominal Heading of the Vehicle is a cardinal direction (e.g., North), these angular separation ranges would approximately correspond to Northwest, Northeast, Southwest, and Southeast. If the established Nominal Heading of the Vehicle is an intercardinal direction (e.g., Northwest), these angular separation ranges would approximately correspond to North, South, East, and West. This arrangement can improve image quality and collection efficiency, particularly when the camera-groups use distortion correcting electronic sensors. In other embodiments, a vehicle with a rotated camera-set travels diagonally while collecting oblique images, improving collection efficiency and image quality.

Biased Emerging Optical Axes

In various embodiments, the emerging optical axes of the cameras in the camera-set are statically biased towards the nominal heading. For example, with four cameras, the emerging optical axes are positioned at 40, 140, 220, and 320 degrees from the nominal heading. This configuration is beneficial because it reduces the impact of the sun on image quality and can extend the time window for collecting oblique imagery.

In some scenarios, this configuration biases the emerging optical axes away from the sun at various times of the day for certain travel patterns (e.g., flying northeast-southwest in the northern hemisphere before solar noon). In other usage models, the biasing reduces glare from the sun that is reflected off the ground (e.g. from water, dew, snow, etc.).

In other scenarios, this biasing reduces the distance between the nominal heading and the inside edges of the swaths of the oblique cameras. This decreases the size of the nadir swath required to have overlap between the nadir and oblique swathes, thereby decreasing the cost and complexity of the nadir camera.

Distortion Correcting Sensors

In various embodiments, the electronic image sensors in the cameras of the camera-set are distortion correcting electronic image sensors. The emerging optical axis of an oblique camera is at an angle to the ground, herein called the down angle, which is typically between 20-70 degrees (but anywhere in the interval (0,90)). As a result of the down angle, the sensor field of view will be distorted when projected through the camera lens to the ground. For example, a rectangular sensor will typically project to a trapezium on the ground. In the special but familiar case of a twist angle equal to zero, a rectangular sensor projects to a trapezoid on the ground. In other scenarios, changes in the medium between the camera and the ground can distort the sensor FOV projection (e.g. if the camera, mounted in air, is capturing an oblique view of the sea bottom through seawater under a horizontal glass window). An example of a distortion correcting sensor is a sensor that reduces this distortion, thereby improving sensor utilization and collection efficiency.

FIG. 12A conceptually illustrates selected details of embodiments of an oblique camera with an electronic image sensor that projects to a distorted sensor field of view. Electronic Image Sensor 1206 is a rectangular, uniform array of pixels organized into rows and columns, e.g. Pixel 1210. The Electronic Image Sensor is contained within the Lens Field 1202, geometrically forming a rectangle inscribed within a circle. In the illustrated embodiment, the oblique camera is part of a Maltese Cross camera-set. When projected to the ground, the Lens FOV 1204 is distorted vertically by the projection from a circle to an ellipse. The Sensor FOV 1208 is similarly distorted from an inscribed rectangle to an inscribed trapezoid. Projected Pixel 1212 is the ground projection of Pixel 1210 and demonstrates the transformation (e.g. a combined vertical and horizontal reflection) caused by the projection.

FIG. 12B conceptually illustrates selected details of embodiments of an oblique camera with a non-uniform distortion correcting electronic image sensor that projects to a corrected sensor field of view. Distortion Correcting Electronic Image Sensor 1226 is a trapezoidal, non-uniform array of pixels organized into rows and columns, e.g. Pixel 1230. The Distortion Correcting Electronic Image Sensor is contained within the Lens Field 1222, geometrically forming a trapezoid inscribed within a circle. In the illustrated embodiment, the oblique camera is part of a Maltese Cross camera-set. When projected to the ground, the Lens FOV 1224 is distorted vertically by the projection from a circle to an ellipse. The Sensor FOV 1228 of the non-uniform pixel array sensor is similarly distorted; however, it is distorted from an inscribed trapezoid to an approximate inscribed rectangle. More specifically, the non-uniform array of pixels is projected to a nearly uniform array of pixels on the ground. Projected Pixel 1232 is the ground projection of Pixel 1230 and demonstrates that in some embodiments, the non-uniform pixel array is designed to almost perfectly cancel the distortion caused by the projection. This enhances the efficiency of oblique imagery collection; the entire swath of the camera is usable because the distortion has been mostly corrected. In various embodiments, the oblique camera is in a non-Maltese Cross configuration (e.g., diagonal).

FIG. 13 conceptually illustrates selected details of embodiments of a diagonal oblique camera with a rotated distortion correcting electronic image sensor that projects to a partially corrected sensor field of view. Rotated Electronic Image Sensor 1306 is a rectangular, uniform array of pixels organized into rows and columns, e.g. Pixel 1310. The Rotated Electronic Image Sensor is contained within the Lens Field 1302, geometrically forming a rectangle inscribed within a circle. However, the Rotated Electronic Image Sensor is rotated around the optical axis of the camera by Twist Angle 1314, which is the angle between the Image Sensor Row Vector 1318 and Horizontal Vector 1316.

Because the oblique camera is projecting diagonally, the Lens FOV 1304 is distorted vertically and horizontally by the projection from a circle to an ellipse. The Sensor FOV 1308 of the rotated sensor is similarly distorted from a rotated inscribed rectangle to a rotated inscribed trapezium. For example, Projected Pixel 1312 is a projection of Pixel 1310 that is distorted. However, the distortion stretches the rotated sensor FOV vertically and horizontally, thereby reducing the horizontal distortion compared to an unrotated sensor. This enhances the efficiency of oblique imagery collection; more of the swath of the camera is usable because the distortion has been reduced. Conceptually, the non-uniform pixel array of FIG. 12B almost perfectly corrects distortion while the Rotated Electronic Image Sensor is a linear approximation of a perfect correction.

In some embodiments, the twist angle of the electronic image sensor is partially determined by the plan and down angles of the oblique camera. In various embodiments, for an oblique camera with plan and down angles of 45 degrees, the twist angle is approximately 53 degrees. This configuration decreases the difference in length between the shortest and longest projected pixel row, improving collection efficiency.

FIG. 14 conceptually illustrates selected details of embodiments of a diagonal oblique camera with a rotated array of rotated distortion correcting electronic image sensors that projects to an array of partially corrected sensor fields of view. The Rotated Array of Rotated Electronic Image Sensors is contained within the Lens Field 1402, geometrically forming a rectangular array inside a circle. Conceptually, a first rotation applies to all electronic image sensors and an individual rotation is also applied to each individual electronic image sensor. Rotated Array of Rotated Electronic Image Sensors 1420 is a rectangular array of multiple image sensors organized into rows. In some embodiments, Rotated Array of Rotated Electronic Image Sensors is a staggered and/or butted array. In FIG. 14, the rows of electronic image sensors are also organized into columns; in other embodiments, the rows of electronic image sensors are staggered. Each electronic image sensor is a rectangular, uniform array of pixels organized into rows and columns, e.g. Rotated Electronic Image Sensor 1410. The entire Rotated Array of Rotated Electronic Image Sensors is rotated around the optical axis of the camera by Twist Angle 1414, which is the angle between the Image Sensor Array Row Axis 1418 and Horizontal Vector 1416. In addition, each rotated electronic image sensor is individually rotated around the optical axis of the camera.

Because the oblique camera is projecting diagonally, the Lens FOV 1404 is distorted vertically and horizontally by the projection from a circle to an ellipse. The Projected Rotated Array of Rotated Electronic Image Sensors 1422 is similarly distorted from a rotated rectangular array to a rotated trapezium array. The sensor FOVs of the rotated sensors within the array (e.g., Sensor FOV 1412) are similarly distorted from rotated rectangles to rotated trapeziums. However, the distortion stretches and shears the rotated array and the rotated sensor FOVs vertically and horizontally, thereby greatly improving the alignment of the e.g. column vectors with the nominal heading compared to an unrotated array of sensors. This enhances the efficiency of oblique imagery collection because more of the swath of the camera is usable. Conceptually, the non-uniform pixel array of FIG. 12B almost perfectly corrects distortion while the Rotated Array of Rotated Electronic Image Sensors is a piece-wise linear approximation of a perfect correction.

In some embodiments, the twist angle is determined by the plan and down angles, and the individual rotations are further determined by the position of each electronic image sensor within the lens field. An individual rotation is determined with reference to a line bisecting the sensor FOV crossing the midpoints of the forward and rear edges of the sensor FOV. The individual rotation is varied until this bisected line is aligned to a common axis (e.g., the nominal heading). In various embodiments, for an oblique camera with 45 degree plan and down angles, the twist angle for the entire array is approximately 53 degrees and the twist angles of the individual sensors relative to the array vary from −10 to +10 degrees.

Rotated electronic image sensors and rotated arrays of rotated electronic image sensors can be used with a variety of oblique cameras, camera-sets, vehicles and nominal headings. For example, one embodiment comprises a vehicle that travels on a nominal heading of approximately 45 degrees from a cardinal direction with four oblique cameras configured with down angles of approximately 45 degrees, and plan angles of approximately 45, 135, 225 and 315 degrees, with the 45 and 225 degree plan angle cameras comprising arrays of rotated image sensors with twist angles of 53 degrees, and with the 135 and 315 degree plan angle cameras comprising arrays of rotated image sensors with twist angles of −53 degrees.

Example Implementation Techniques

In various embodiments the vehicle is an airplane, helicopter, lighter-than-air craft, boat, ship, barge, submersible, satellite, space-craft, car, or truck. These might be manned or unmanned.

In some embodiments, rather than having a single electronic image sensor behind each camera lens, a mosaic of several sensors is used. This mosaic is assembled at a single Petzval surface at the rear of the lens. In other embodiments, the lens admits light through a series of partially reflecting surfaces, so that the image sensors are assembled onto multiple surfaces, with the active areas overlapping. In various embodiments, the partially reflecting surfaces are spectrally selective, in order to use the different sensors to capture different portions of the electromagnetic spectrum. In some embodiments, the partially reflective surfaces are polarization selective, in order to use the different sensors to capture the polarization information of the incoming light. In yet other embodiments, the reflecting surfaces simply divide the incoming light evenly between multiple Petzval surfaces. In various embodiments, the mosaic comprises several line-format sensors, each collecting light from different portions of the spectrum.

In some embodiments, a mosaic of line-format sensors is employed at the forward and rear edges of the field of view of the lenses, so that the same points on the ground are collected from view angles approximately 10 degrees apart, at times separated by a few seconds. To capture a combination of depth and spectral information, each lens carries behind it a mosaic of both line-format and area-format sensors. The resulting images are useful for extracting 3D depth information from the scene.

CONCLUSION

Certain choices have been made in the description merely for convenience in preparing the text and drawings and unless there is an indication to the contrary the choices should not be construed per se as conveying additional information regarding structure or operation of the embodiments described. Examples of the choices include: the particular organization or assignment of the designations used for the figure numbering and the particular organization or assignment of the element identifiers (the callouts or numerical designators, e.g.) used to identify and reference the features and elements of the embodiments.

The words “includes” or “including” are specifically intended to be construed as abstractions describing logical sets of open-ended scope and are not meant to convey physical containment unless explicitly followed by the word “within.”

Although the foregoing embodiments have been described in some detail for purposes of clarity of description and understanding, the invention is not limited to the details provided. There are many embodiments of the invention. The disclosed embodiments are exemplary and not restrictive.

It will be understood that many variations in construction, arrangement, and use are possible consistent with the description, and are within the scope of the claims of the issued patent. The order and arrangement of flowchart and flow diagram process, action, and function elements are variable according to various embodiments. Also, unless specifically stated to the contrary, value ranges specified, maximum and minimum values used, or other particular specifications (such as number and configuration of cameras or camera-groups, number and configuration of electronic image sensors, nominal heading, down angle, twist angles, and/or plan angles), are merely those of the described embodiments, are expected to track improvements and changes in implementation technology, and should not be construed as limitations.

Functionally equivalent techniques known in the art are employable instead of those described to implement various components, sub-systems, operations, functions, routines, sub-routines, in-line routines, procedures, macros, or portions thereof.

The embodiments have been described with detail and environmental context well beyond that required for a minimal implementation of many aspects of the embodiments described. Those of ordinary skill in the art will recognize that some embodiments omit disclosed components or features without altering the basic cooperation among the remaining elements. It is thus understood that much of the details disclosed are not required to implement various aspects of the embodiments described. To the extent that the remaining elements are distinguishable from the prior art, components and features that are omitted are not limiting on the concepts described herein.

All such variations in design are insubstantial changes over the teachings conveyed by the described embodiments. It is also understood that the embodiments described herein have broad applicability to other imaging, survey, surveillance, and photogrammetry applications, and are not limited to the particular application or industry of the described embodiments. The invention is thus to be construed as including all possible modifications and variations encompassed within the scope of the claims of the issued patent. 

What is claimed is:
 1. A method comprising: capturing complete coverage of a collection area via successive overlapping frames of oblique down-angle imagery along successive nominally parallel collection lines separated by a collection line pitch in accordance with a swath of adjacent strips of composite imagery of the overlapping frames; and operating at least one camera-group with at least one optical axis nominally rotated by a predetermined oblique plan angle relative to a nominal axis of travel to achieve a corresponding predetermined swath width greater than achieved by operating the at least one optical axis nominally parallel to the nominal axis of travel.
 2. The method of claim 1, wherein the at least one camera-group is comprised in a flying vehicle, the collection lines are flight lines and the collection line pitch is a flight line pitch.
 3. The method of claim 1, wherein for a same collection line orientation, total distance traveled to obtain the complete coverage is reduced compared to total distance required by operating the at least one optical axis nominally parallel to the nominal axis of travel.
 4. The method of claim 1, wherein the nominal rotation of the at least one optical axis is further adjusted for at least one reason other than achieving the predetermined swath width.
 5. The method of claim 1, wherein the predetermined oblique plan angle is at least one of approximately 45, 135, 225, and 315 degrees.
 6. The method of claim 1, wherein the predetermined swath width is approximately 30% greater than that achieved by operating the at least one optical axis nominally parallel to the nominal axis of travel.
 7. The method of claim 1, wherein the collection lines are nominally in an intercardinal direction.
 8. The method of claim 7, wherein the operating results in the at least one optical axis being oriented in a nominally cardinal direction during the capturing.
 9. The method of claim 1, further comprising: operating the at least one camera-group with an electronic image sensor nominally rotated around the at least one optical axis by a predetermined nominally non-zero angle with respect to a non-rotated position.
 10. The method of claim 9, wherein: the predetermined swath width is a first predetermined swath width and the non-zero rotation of the electronic image sensor enables achieving a corresponding second predetermined swath width greater than the first predetermined swath width; and to obtain the complete coverage for a same collection line orientation, total distance traveled is reduced while operating in accordance with the second predetermined swath width compared to total distance required by operating in accordance with the first predetermined swath width.
 11. The method of claim 10, wherein the second predetermined swath width corresponds to an entire swath of the camera-group, while the first predetermined swath width is an effectively usable subset of the entire swath.
 12. The method of claim 9, wherein projected linear sets of pixel elements of the electronic image sensor are aligned to reduce angular separation with respect to the nominal axis of travel and the projected linear sets.
 13. The method of claim 12, wherein the angular separation is below a pre-determined separation threshold.
 14. A system comprising: means for capturing complete coverage of a collection area via successive overlapping frames of oblique down-angle imagery along successive nominally parallel collection lines separated by a collection line pitch in accordance with a swath of adjacent strips of composite imagery of the overlapping frames; and means for operating at least one camera-group with at least one optical axis nominally rotated by a predetermined oblique plan angle relative to a nominal axis of travel to achieve a corresponding predetermined swath width greater than achieved by operating the at least one optical axis nominally parallel to the nominal axis of travel.
 15. The system of claim 14, wherein for a same collection line orientation, total distance traveled to obtain the complete coverage is reduced compared to total distance required by operating the at least one optical axis nominally parallel to the nominal axis of travel.
 16. The system of claim 14, wherein the predetermined oblique plan angle is at least one of approximately 45, 135, 225, and 315 degrees.
 17. The system of claim 14, wherein the collection lines are nominally in an intercardinal direction.
 18. The system of claim 14, further comprising: means for operating the at least one camera-group with an electronic image sensor nominally rotated around the at least one optical axis by a predetermined nominally non-zero angle with respect to a non-rotated position.
 19. The system of claim 18, wherein: the predetermined swath width is a first predetermined swath width and the non-zero rotation of the electronic image sensor enables achieving a corresponding second predetermined swath width greater than the first predetermined swath width; and to obtain the complete coverage for a same collection line orientation, total distance traveled is reduced while operating in accordance with the second predetermined swath width compared to total distance required by operating in accordance with the first predetermined swath width.
 20. The system of claim 19, wherein the second predetermined swath width corresponds to an entire swath of the camera-group, while the first predetermined swath width is an effectively usable subset of the entire swath.
 21. The system of claim 18, wherein projected linear sets of pixel elements of the electronic image sensor are aligned to reduce angular separation with respect to the nominal axis of travel and the projected linear sets.
 22. The system of claim 21, wherein the angular separation is below a pre-determined separation threshold. 